Allow for negative top/left offsets in composite overlays

A top or left offset value of -1 will no longer mean that the
value is not set, but will now be an actual offset of -1.

INT_MIN for left & top will mean that the values are not set.

Co-authored-by: Christian Flintrup <chr@gigahost.dk>

src/common.cc

@@ -658,26 +658,18 @@ namespace sharp {
     int top = 0;
 
     // assign only if valid
-    if (x >= 0 && x < (inWidth - outWidth)) {
+    if (x < (inWidth - outWidth)) {
       left = x;
     } else if (x >= (inWidth - outWidth)) {
       left = inWidth - outWidth;
     }
 
-    if (y >= 0 && y < (inHeight - outHeight)) {
+    if (y < (inHeight - outHeight)) {
       top = y;
     } else if (y >= (inHeight - outHeight)) {
       top = inHeight - outHeight;
     }
 
-    // the resulting left and top could have been outside the image after calculation from bottom/right edges
-    if (left < 0) {
-      left = 0;
-    }
-    if (top < 0) {
-      top = 0;
-    }
-
     return std::make_tuple(left, top);
   }
 

src/pipeline.cc

@@ -570,7 +570,7 @@ class PipelineWorker : public Napi::AsyncWorker {
               int left;
               int top;
               compositeImage = compositeImage.replicate(across, down);
-              if (composite->left >= 0 && composite->top >= 0) {
+              if (composite->hasOffset) {
                 std::tie(left, top) = sharp::CalculateCrop(
                   compositeImage.width(), compositeImage.height(), image.width(), image.height(),
                   composite->left, composite->top);
@@ -592,7 +592,7 @@ class PipelineWorker : public Napi::AsyncWorker {
           // Calculate position
           int left;
           int top;
-          if (composite->left >= 0 && composite->top >= 0) {
+          if (composite->hasOffset) {
             // Composite image at given offsets
             std::tie(left, top) = sharp::CalculateCrop(image.width(), image.height(),
               compositeImage.width(), compositeImage.height(), composite->left, composite->top);
@@ -1253,6 +1253,7 @@ Napi::Value pipeline(const Napi::CallbackInfo& info) {
     composite->gravity = sharp::AttrAsUint32(compositeObject, "gravity");
     composite->left = sharp::AttrAsInt32(compositeObject, "left");
     composite->top = sharp::AttrAsInt32(compositeObject, "top");
+    composite->hasOffset = sharp::AttrAsBool(compositeObject, "hasOffset");
     composite->tile = sharp::AttrAsBool(compositeObject, "tile");
     composite->premultiplied = sharp::AttrAsBool(compositeObject, "premultiplied");
     baton->composite.push_back(composite);

src/pipeline.h

@@ -40,6 +40,7 @@ struct Composite {
   int gravity;
   int left;
   int top;
+  bool hasOffset;
   bool tile;
   bool premultiplied;
 
@@ -47,8 +48,9 @@ struct Composite {
     input(nullptr),
     mode(VIPS_BLEND_MODE_OVER),
     gravity(0),
-    left(-1),
-    top(-1),
+    left(0),
+    top(0),
+    hasOffset(false),
     tile(false),
     premultiplied(false) {}
 };